High intensity sweeteners possess a sweetness level many times exceeding that of sucrose. They are essentially non-caloric and used widely in manufacturing of diet and reduced calorie food. Although natural caloric sweeteners such as sucrose, fructose, and glucose provide the most desirable taste to consumers, they possess high calorie values. High intensity sweeteners do not affect the blood glucose level and provide little or no nutritive value.
Stevia rebaudiana Bertoni is a perennial shrub of the Asteraceae (Compositae) family native to certain regions of South America. The leaves of the plant contain from 10 to 20% of diterpene glycosides, which are around 150 to 450 times sweeter than sugar. The leaves have been traditionally used for hundreds of years in Paraguay and Brazil to sweeten local teas and medicines.
At present there are more than 230 Stevia species with significant sweetening properties. The plant has been successfully grown under a wide range of conditions from its native subtropics to the cold northern latitudes.
The extract of Stevia rebaudiana plant contains a mixture of different sweet diterpene glycosides, which have a single base—steviol—and differ by the presence of carbohydrate residues at positions C13 and C19. These glycosides accumulate in Stevia leaves and compose approximately 10%-20% of the total dry weight. Typically, on a diy weight basis, the four major glycosides found in the leaves of Stevia are Dulcoside A (0.3%), Rebaudioside C (0.6-1.0%), Rebaudioside A (3.8%) and Stevioside (9.1%). Other glycosides identified in Stevia extract include Rebaudioside B, C, D, E, and F, Steviolbioside and Rubusoside. Among steviol glycosides only Stevioside and Rebaudioside A are available on a commercial scale.
Steviol glycosides have zero calories and can be used wherever sugar is used. They are ideal for diabetic and low calorie diets. In addition, the sweet steviol glycosides possess functional and sensory properties superior to those of many high potency or high intensity sweeteners.
Rebaudioside D (CAS No: 63279-13-0), as shown in FIG. 1, is one of the sweet glycosides found in Stevia rebaudiana. Studies show that highly purified forms of Rebaudioside D possess a very desirable taste profile, almost lacking the bitterness and lingering licorice aftertaste typical for other Steviol glycosides.
These properties multiply the significance of Rebaudioside D and attract great interest for methods of preparation of highly purified forms of Rebaudioside D. However, highly purified steviol glycosides possess relatively low water solubility. For example Rebaudioside A thermodynamic equilibrium solubility at room temperature is only 0.8%.
On the other hand, it is well known that Rebaudioside A exhibits so called polymorphism (Zell T. M., Padden B. E., Grant D. J. W., Schroeder S. A., Wachholder K. L., Prakash I., Munsona E. J. (2000) Investigation of Polymorphism in Aspartame and Neotame Using Solid-State NMR Spectroscopy, Tetrahedron, 56, 6603-6616). Rebaudioside A amorphous, anhydrous and solvate forms differ significantly from each other in terms of solubility, which is one of the main criteria for the commercial viability of a sweetener. In this regard, as shown in Table 1, the hydrate form of Rebaudioside A, displays the lowest solubility (Prakash I., DuBois G. E., Clos J. F., Wilkens K. L., Fosdick L. E. (2008) Development of rebiana, a natural, non-caloric sweetener, Food Chem. Toxicol., 46, S75-S82). It was shown that Rebaudioside A may transform from one polymorph form to another at certain conditions (U.S. patent application Ser. No. 11/556,049).
TABLE 1Properties of Rebaudioside A forms (U.S. patent application Ser. No. 11/556,049)Polymorph FormsForm 1 Form 2Form 3 Form 4HydrateAnhydrousSolvateAmorphousRate ofVery lowIntermediateHigh High dissolution in(<0.2% in(<30% in (>30% in (>35% inH2O at 25° C.60 minutes)5 minutes)5 minutes)5 minutes)Alcohol content<0.5%<1%1-3%<0.05%Moisture  >5%<1% <3% 6.74%content
Rebaudioside D possesses even lower water solubility compared to Rebaudioside A. In room temperature it can be dissolved only at 0.05%. When heat is applied, one can make up to 0.5% solution, but upon cooling to room temperature, Rebaudioside D will quickly crystallize back out from the solution. Considering high sweetness intensity of Rebaudioside D, even 0.05% solubility can be sufficient for many applications.
Many food production processes use highly concentrated ingredient mixes prior to producing final forms of food products. In that case, higher concentrations of dissolved Rebaudioside D will be required. It has to be noted that using the heat for dissolution of Rebaudioside D may not be possible in many compositions which contain heat sensitive components. Also maintaining high temperature of mixture for prolonged time to prevent premature crystallization of Rebaudioside D can cause thermal degradation of mixture components or undesirable changes of organoleptic properties.
Therefore there is a need for developing highly soluble forms or compositions of Rebaudioside D which can provide stable solutions with minimal or no heat treatment.
Furthermore, considering the similar chemical structures of Rebaudioside D and other steviol glycosides, as well as other terpene glycosides, the developed approaches may be used in the case of other glycosides as well.